Keywords

1 Introduction

Shape Memory Alloys (SMA) such as NiTi have enabled technology development in various areas, such as microrobotics and manipulation for instance. These alloys undergo a reversible solid-state displacive crystalline and phase transformation dominated by shear between a high symmetry parent phase (austenite in form of ordered BCC superlattice \(\upbeta \) phase in the case of Ni-50.0 % Ti) and a low symmetry product phase (martensite in the form of monoclinic distortion of a B19 lattice) [1].

Using titanium and alloys in biomedical productions continues to gain note, because of Titanium and alloys unique properties, including high specific strenght, low density and lightweight feel, excellenet corrosion resistance, and biocompatility [2]. Nickel Titanium (also known as Nitinol or NiTi) is in a unique class of shape memory alloys. A thermoelastic martensitic phase transformation which the material contents is responsible for its extraordinary properties. Nitinol properties include the shape memory effect, superelasticity, and high damping capability. These technical properties of NiTi shape memory alloys can be modified to a great extent by changes in composition, mechanical working, and heat treatment [3].

Ni–Ti binary equilibrium was seen in Fig. 18.1. In phase diagram thermodynamically stable phases exist in the proximity of equiatomic percentages of Ni and Ti. Applying heat treatments can have significant effects on types of microstructural phases in the final products and also on thermomechanical properties of NiTi devices, such as annealing, solution treatment, and aging [4].

Fig. 18.1
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Ni–Ti phase diagram [5]

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Many applications require material properties which polymers cannot provide e.g. if high strenght, high corrosion or thermal resistance are required. This can be solved by powder injection molding which is an economically viable process for complex shaped metal or ceramic parts in large scale series [6].

For this purpose, metal or ceramic powders are mixed with a binder system and injected into the mold which contains the microstructured mold inserts. In order to achive a good filling of the mold inserts, feedstocks of low viscosity and evacuated injection molding are required. Normally, a relatively high mechanical stability of the binder system is necessary for a safe demolding. The molded part is processed further in a furnace to remove the binder system. Subsequently, sintered under defined athmosphere to achive a dense micro component [6].

In this study, NiTi shape memory alloy powders were investigated to productubility of Powder Injection Moulding (Fig. 18.2). For this porpose, powders were analized for size, DSC and EDS analizer.

Fig. 18.2
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Schematic diagram of MIM process [7]

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2 Experimental Study

2.1 Powder Characterization

In this section, prealloyed Ni-rich TiNi powder (Ti-55.52 % at Ni 99.9 % purity, supplied by Nanoval GmbH and Co.KG) were used as the raw material. Two sized powders were obtained to fabric. These powders general structure were examined by using SEM images (Figs. 18.3 and 18.4). The examinationed of the overall structure of powders founded spherical in SEM images. For having a good productubility, powders were spherical and sized 35 and 10 \(\upmu \)m with a low sattallites.

Fig. 18.3
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SEM images of 10 \(\upmu \)m sized powders

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Fig. 18.4
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SEM images of 35 \(\upmu \)m sized powders

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Fig. 18.5
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NiTi powders average particle size (10 \(\upmu \)m)

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Fig. 18.6
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NiTi powders average particle size (35 \(\upmu \)m)

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Powders were analyzed with the size analizer and could be seen there that aproximately the size was given by manufacturer. While this fabricate gave in receipt 10, 13 \(\upmu \)m was founded by powder size analizer (Fig. 18.5) and 36.17 \(\upmu \)m was founded for 35 \(\upmu \)m powders by powder size analizer (Fig. 18.6).

In order to determine the transformation temperature of the SMA, DSC analysis were applied on 10 and 35 \(\upmu \)m sized-powders (Figs. 18.7 and 18.8). The onset temperature of the alloy was at \(-24.35\,^{\circ }\)C and the end temperature was at \(-1.97\,^{\circ }\)C. For 35 \(\upmu \)m sized powders onset and final temperature was between \(-15.17\) and 4.17 \(^{\circ }\)C which could be seen in Fig. 18.4.

Fig. 18.7
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DSC analysis of the 10 \(\upmu \)m size powders

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Fig. 18.8
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DSC analysis of the 35 \(\upmu \)m size powders

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Fig. 18.9
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SEM micrograph of the NiTi powders sized 10 \(\upmu \)m

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Fig. 18.10
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10 \(\upmu \)m-sized powders a and b EDS results

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Fig. 18.11
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SEM micrograph of the NiTi powders sized 35 \(\upmu \)m

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Fig. 18.12
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35 \(\upmu \)m-sized powders a, b and c EDS results

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To obtain the chemical composition of NiTi powders, microanalsis was carried out by using EDS (Figs. 18.10 and 18.12) on the SEM images (Figs. 18.9 and 18.11). The spot analyses (Figs. 18.10 and 18.12) revealed two structure, Ni and Ti being present in both. According to the EDS peaks, only Ni and Ti peaks were determined. Ni ratio was more than Ti ratio for 10 \(\upmu \)m sized powders. When compared Figs. 18.10 and 18.12, Ti ratio was more than in Fig. 18.12.

The binder which was used in this work is composite of 80 % PEG 8000 (Sigma Aldrich), 15 % Polyproplen (Petkim Co.Inc.) and 5 % Stearic Asit (SA). Powder loading for this work is 50, 60 and 70 % and binder used. For first binder system, it was composed of PEG 8000, Poliproplen and Stearik Asit. The first peak which was given by PEG 8000 and Stearik Asit (peak temperature was 69.85 \(^{\circ }\)C). Poliproplen’s peak was the second and approximately 161.90 \(^{\circ }\)C (Fig. 18.13). Second binder’s tranformation temperature was aproximately the same as the first temperature (Fig. 18.14).

Fig. 18.13
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DSC analysis of Binder 1

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Fig. 18.14
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DSC analysis of Binder 2

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It can be seen that in TG result (Fig. 18.15), material loss was beginnig approximately 205 \(^{\circ }\)C. DTA results showed that aproximately at 70 and 170 \(^{\circ }\)C two transformation obtained. First one is PEG 800 and SA, second one is Poliproplen.

Fig. 18.15
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DTG, DTA and TG analysis of Binder 1

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3 Conclusion

Our proposal, avoiding the exothermic reaction problems with elemental Ni and Ti powders, prealloyed NiTi powders (diameter \(<\)20 \(\upmu \)m) were used [8]. Prealloyed NiTi powders were used and investigated for mouldability in powder injection moulding method. For productubility, two types of powders were used. Differential Scanning Calorimetry (DSC) results of the prealloyed NiTi powders (approximately; Ni55–Ti) indicate that martensitic transition occurs between \(-15\) and 4 \(^{\circ }\)C for 35 \(\upmu \)m sized powders and between \(-24\) and \(-2\,^{\circ }\)C for 10 \(\upmu \)m sized powders. NiTi alloys generally used for manufacturing implants and powder metallurgy process is the most versatile and practical ones to make NiTi implants were also known [8]. Two type of binders were investigated and used in rheolojical application, including PEG 8000, Poliproplen, Stearik Acid. High quality parts could be produced for medical application with powder injection moulding.

(With powder injection moulding, could be produced high quality parts for medical application)